1. Field of the Invention
This invention relates to a method for the production of methacrylic acid. More particularly, it relates to a method for producing methacrylic acid by subjecting at least one compound selected from the group consisting of methacrolein and isobutyl aldehyde or at least one compound selected from the group consisting of isobutylene, t-butanol, and methyl-t-butyl ether to catalytic vapor-phase oxidation using a molecular oxygen-containing gas.
2. Description of the Prior Art
Heretofore, for the production of methacrylic acid by catalytic vapor-phase oxidation of isobutylene, t-butanol, or methyl-t-butyl ether, the so-called two-stage oxidation reaction which comprises provisionally converting isobutylene, t-butanol, or methyl-t-butyl ether into methacrolein by catalytic vapor-phase oxidation in the presence of a catalyst (this reaction and the catalyst used therein will be occasionally referred to hereinafter as "former-stage reaction" and "former-stage catalyst" respectively) and subsequently converting methacrolein into methacrylic acid by catalytic vapor-phase oxidation in the presence of a catalyst (this reaction and the catalyst used therein will be occasionally referred to hereinafter as "latter-stage reaction" and "latter-stage catalyst" respectively) has been generally adopted. The oxide catalyst containing bismuth, molybdenum, and iron has been generally used as the former-stage catalyst and the oxide catalyst containing molybdenum and phosphorus as the latter-stage catalyst respectively in the two-stage reaction.
The method of producing methacrylic acid by subjecting isobutyl aldehyde to catalytic vapor-phase oxidative dehydrogenation and oxidation in the presence of the latter-stage catalyst has been also adopted.
The methacrylic acid which is produced as described above is generally subjected to a purifying treatment and then commercially utilized as a raw material for the production of an alkyl methacrylate by the reaction of esterification with an alcohol of 1 to 12 carbon atoms, for example. The alkyl methacrylate which is thus produced is commercially utilized as a raw material for the production of a polyalkyl methacrylate by homopolymerization or copolymerization with an other monomer.
The alkyl methacrylate produced by using as the raw material the methacrylic acid obtained by the conventional method described above, however, contains impurities in a minute amount. In all these impurities, the furan type compounds are considered to form one cause for the coloration of the polyalkyl methacrylate which is obtained by the polymerization of the alkyl methacrylate.
Particularly in the case of polymethyl methacrylate as a general-purpose polymer among other polyalkyl methacrylates, the coloration due to the presence of the furan compounds occurs to a large degree. By this reason, various measures are taken to enhance the transparency of the polymethyl methacrylate by removing such furan compounds.
The furan compounds which are contained as impurities in the methyl methacrylate are considered to originate from trace of impurities, particularly diketones, contained in the methacrylic acid as the raw material. For the purpose of decreasing the content of furan compounds in the methyl methacrylate, therefore, not only the decrease of the content of furan compounds in methyl methacrylate by the purifying treatment but also the decrease of the content of diketones in the methacrylic acid as the raw material is an indispensable requirement. Heretofore, for the purpose of decreasing the content of diketones in the aqueous methacrylic acid solution obtained from the methacrylic acid absorption column, methods of increasing the ratio of extraction of diketones as by varying the kind of the solvent to be used in the step of solvent extraction which is one of the steps for purification have been adopted.
According to our knowledge, while the gas containing methacrolein and/or methacrylic acid contains some ten to some hundred mol ppm, occasionally some thousand mol ppm, of diketones, based on the amount of the compound as raw material, the aqueous methacrylic acid solution obtained from the methacrylic acid absorption column still contains some ten to some hundred mol ppm of diketones based on the amount of methacrylic acid even after extraction from the solvent. As already described, the diketones form a cause for the formation of the furan type compounds. For the purpose of repressing the content of furan compounds in the ester such as methyl methacrylate to some ten mol ppm, it is necessary that the content of diketones in methacrylic acid should be also repressed to some ten mol ppm. Under the present condition, the relevant reaction products are in need of further purification.
When the number of steps of purification is increased, however, a serious economic disadvantage arises in respect that the loss of methacrylic acid is increased, the utility involved in purification is increased, and the cost of production of methacrylic acid or the ester thereof is proportionately boosted.
Further, in the production of methacrylic acid, when the content of diketones in methacrylic acid is increased by aging or by a change in the reaction condition of the step of oxidation, for example, the sole operation of removing impurities in the step of purification does not deserve to be called sufficient and, therefore, calls for an additional measure to decrease the content of diketones.
An object of this invention is to provide a novel method for the production of methacrylic acid.
Another object of this invention is to provide a method for very simple and inexpensive production of methacrylic acid, which method curbs formation of diketones as by-products and allows production of methacrylic acid having an extremely small content of diketones and enjoying high quality.